It is known to detect by interferometer variations in distances between two objects, in that at one of the objects a retroreflector is mounted, and at the other one of the two objects an emitter for coherent radiation is provided which is directed to the retroreflector. The radiation retroreflected by the retroreflector towards the other object is received via a respective receiver, made to superimpose the radiation of the source by interference and detected by means of a radiation intensity sensor. In the case of a change in the distance between the two objects, the optical path length of the beam reflected back and forth between the two objects, so that due to the interference intensity maximums and minimums are generated. If the number of intensity variations is detected, a conclusion can be drawn to the magnitude of the variation in distance. A measurement of the absolute distance between the two objects is possible, if in a reference run the two objects are initially placed in a predetermined known distance from each other and then the variations from the known distance are detected.
From WO 88/07656 a system is known for detecting relative changes in position and orientation of two objects relative to each other. For this purpose, the retroreflectors are mounted on the first object at a distance from each other, and on the second object six separate interferometer-emitters are provided at a distance from each other, the radiation of each of which is directed to one of the retroreflectors. Herein the first, second and third emitters each direct their radiation to the first retroreflector, the fourth and the fifth emitters direct their radiation to the second retroreflector and the sixth emitter directs its radiation to the third retroreflector. Thus, by providing the six interferometers, a variation of six different distances between locations on the first or second object, respectively, can be detected. If the distance between the different locations on the two objects in a reference position was known at the beginning of a measurement, then after displacing the two objects relative to each other the six distances can be determined absolutely. Then, from the absolute values of the six distances, the position and the orientation of the two objects relative to each other can be determined.
If an interruption of one of the six beams occurs in this system during a variation in the distance or orientation, respectively, for example due to environmental influences, the intensity maximums or minimums, respectively, can no longer be counted, so that a determination of the absolute distance between the two objects is no longer possible from this point in time. Further, each emitter must direct its radiation to that only retroreflector which is assigned to it exclusively, so that the emitter comprises a corresponding beam direction control, in order to maintain its radiation constantly directed to the corresponding retroreflector. In the case of beam interruption, the direction control can lose contact with the corresponding retroreflector.
If the coherent radiation is supplied to the emitter by a conventional one-mode light conductor and finally by the latter is supplied to the detector, a change in the transmission properties of the light conducting fiber occurs also in the case of a variation of the relative position between source and detector on the one hand and emitter on the other hand, or by other environmental influences. A consequence thereof may be that the signal supplied to the detector is subjected to strong fluctuations, which are also interpreted as intensity maximums or minimums, respectively, caused by movement between the two objects. This also leads to faulty measurement of the distance variation.
From the six detected distances, the determination of the position and the orientation of the two objects relative to each other by means of an iterative calculation process is possible, such as a conventional Gauss-Seidel method. Such a method requires much calculation and limits the speed with which the detected position or orientation, resp., per time unit can be updated.